1. Introduction
This invention relates to aqueous dispersions of conductive particles. More particularly, this invention relates to stabilizing said dispersions using a polymeric surfactant having at least three alkylene oxide repeating units and a hydrophilic--lipophilic balance of at least 10.
2. Description of the Prior Art
Dispersions of conductive particles for formation of conductive coatings are known in the art. The conductive coatings formed using these dispersions are used for a variety of purposes such as a base for electroplating, for electromagnetic shielding and discharge, to make resistance heaters, as reflectors for radar dishes, as conductor paths for circuitry, etc. Typical dispersion of conductive particles include dispersions of carbon and graphite as illustrated in U.S. Pat. Nos. 4,818,437, 4,834,910, 4,874,477 and 5,476,580; metals such as silver, copper and palladium as illustrated in U.S. Pat. No. 5,500,106; metal salts such as metal oxides and sulfides as disclosed in U.S. Pat. Nos. 5,268,024 and 5,269,973; and organic polymers such as those illustrated in U.S. Pat. No. 5,415,762. Each identified patent is incorporated herein by reference for its disclosure of dispersions of conductive particle dispersions.
The use of coatings formed from dispersions of conductive particles for the formation of printed circuit boards is disclosed in U.S. Pat. No. 3,099,608, incorporated herein by reference. The process disclosed in this patent involves treatment of a nonconducting surface with a tin-palladium dispersion to form a poorly conducting film of colloidal palladium particles over the nonconducting surface. This is the same tin-palladium colloid used as a plating catalyst for electroless metal deposition. For reasons not fully understood, it is possible to electroplate directly over the catalyzed surface of the nonconductor from an electroplating solution though deposition occurs by propagation and growth from a conductive surface, such as the copper cladding used in the fabrication of a printed circuit board. Therefore, deposition begins at the interface of an adjacent conductive surface and the catalyzed nonconducting surface. The deposit grows epitaxially along the catalyzed surface from this interface. Metal deposition onto the substrate using this process is slow. Moreover, deposit thickness is uneven with the thickest deposit occurring at the interface with the conducting surface and the thinnest deposit occurring at a point most remote from the interface.
It is stated that an improvement to the process of U.S. Pat. No. 3,099,608 is described in UK Patent No 2,123,036 B, incorporated herein by reference. In accordance with the process described in this patent, a surface is provided with metallic sites by immersion in a dispersion of conductive particles such as metallic particles inclusive of tin-palladium colloidal particles. The surface is then electroplated from an electroplating solution containing an additive said to inhibit deposition of metal on a metal deposit formed during the plating reaction without inhibiting deposition on the metallic sites over the nonconducting surface. This is said to result in preferential deposition over the metallic sites with a concomitant increase in the overall plating rate. A commercial application of the process of the UK patent is for the metallization of through-holes walls in the manufacture of double sided printed circuit boards.
Further improvements in processes for direct electroplating of nonconductors in printed circuit manufacture are disclosed in U.S. Pat. Nos. 4,895,739; 4,919,768 and 4,952,286, each incorporated herein by reference. In accordance with the processes of these patents, a palladium dispersion, such as that disclosed in the UK patent, is treated with an aqueous solution of a chalcogen, such as a solution of a sulfur compound, to convert the catalytic surface to a chalcogenide surface. The chalcogenide of the electroless plating catalyst formed by conversion of the surface to a chalcogenide conversion coating is robust and suitable for electroplating. A shortened process for electroplating derived from the teachings of these patents and using a chalcogenide is disclosed in U.S. Pat. No. 5,276,290 incorporated herein by reference. In accordance with the process disclosed in this patent, a stable dispersion of a preformed catalytic chalcogenide is prepared. A non-conducting surface is contacted with the dispersion by immersion. Chalcogenide particles adsorb on the surface of the nonconductor. Following adsorption of the chalcogenide, the non-conductor is electroplated following procedures disclosed in the aforesaid U.S. Pat. Nos. 4,895,739; 4,919,768 and U.S. Pat. No. 4,952,286.
An alternative method for direct electroplating of nonconductors is disclosed in U.S. Pat. No. 4,619,741, incorporated herein by reference. In accordance with the procedures of this patent, a nonconductive substrate is coated with a dispersion of carbon black and then dried. The coating is removed from surfaces where plating is undesired and the remaining portions of the substrate are plated using procedures similar to those described in the aforesaid references. There are several problems inherent in this procedure. Carbon black is a poor conductor of electricity, and consequently, before forming the carbon black dispersion, in practice, it is believed that the carbon black particles must be treated with an organic ionomer or polymer to enhance conductivity. In addition, during processing, and prior to electroplating, the carbon black dispersion is only poorly adhered to the underlying substrate and has a tendency to flake off of the substrate prior to the plating step. This results in void formation during plating. In addition, because of the poor adherence to the substrate, subsequent to plating, there is a tendency for the metal deposit to separate from the substrate. This can lead to interconnect defects between a metallized hole and an innerlayer in multilayer printed circuit fabrication.
A more recently utilized direct plate process for metallizing the walls of hole-walls employs dispersions of graphite for the formation of a conductive coating. The use of graphite to form conductive coatings on through-hole walls is known and disclosed in U.S. Pat. No. 2,897,409 incorporated herein by reference. Current processes are disclosed, for example, in U.S. Pat. Nos. 4,619,741 and 5,389,270, each incorporated herein by reference. In accordance with the procedures of these patents, a dispersion of carbon black or graphite is passed through the through-holes of a printed circuit board substrate to form a coating of the dispersion on the hole-walls. The coating is dried to yield a conductive layer of the carbon black or graphite which is sufficiently conductive for electroplating in a conventional manner. In both the carbon and graphite processes, because of the poor conductivity of these materials relative to a metal, there is a tendency for the metal deposit to propagate from a conductive surface such as copper cladding in the fabrication of printed circuit boards. Therefore, the thickness of a deposit is uneven, thicker in the areas in closest proximity to the metallic layer and thinnest in the area most remote from the metallic surface. In addition, take-off time for initiation of deposition can be relatively slow, again as a consequence of the relatively poor conductivity of the carbonaceous coating.
In U.S. Pat. Nos. 5,484,518 and 5,500,106, each incorporated herein by reference, an alternative to the use of carbon and graphite dispersions is disclosed. In accordance with the procedure described in these patents, a dispersion of other conductive particles is used such as particles of metal, conductive metal salts and conductive oxides. As further disclosure within these patents, additional components are added to the dispersion such as conductive polymers, binders, surfactants and reducing agents.
Though the process of the aforesaid patents provides conductive coatings, in practice it has been found that due to the relatively large particle size of the particles in the dispersion, especially the carbon and graphite dispersions, the dispersions used are most often inadequately stable for prolonged storage prior to use. This limits the market for the dispersions or requires the addition of a stabilizer to stabilize the dispersion. It has been found that when a stabilizer is added to the dispersion to enhance stability, the stabilizer envelopes the particles within the dispersion and form an electrically insulative coating over the particles. Therefore, when the particles are coated onto a substrate, the particles which are initially poorly conductive, become less conductive because of the insulating layer of the stabilizer thereby further limiting the commercial use of the same.